The present invention relates to contaminant treatment systems, and more particularly to a system and method for photocatalytic treatment of contaminated media.
Treatment systems and techniques for removing contaminants from contaminated gas have been developed in the past. Some of these treatment systems and techniques include treatments using a photocatalytic process. Common photocatalytic treatment methods typically make use of a technique by which a photocatalyst is bonded or fixed to a substrate or a fluidized bed reactor. Photocatalytic reactions are then caused by irradiating electromagnetic radiation such as ultraviolet light on the fixed photocatalyst so as to activate it. Resulting photocatalytic reactions bring about destruction of contaminants, such as volatile organic contaminants or other biologically harmful compounds that are in close proximity to the activated photocatalyst.
Several significant problems may be encountered when using known photocatalytic air treatment systems and techniques. One problem is the need to maximize xe2x80x9cmass transfer,xe2x80x9d which is the number of collisions between contaminants and an activated photocatalyst. Photocatalytic destruction occurs only at the surface of the photocatalyst where the photocatalyst contacts the contaminants. If an insufficient level of mass transfer occurs, contaminants will pass directly through the treatment system without being subjected to the photocatalyst. Consequently, known photocatalytic air treatment systems are significantly oversized to overcome mass transfer limitations, which reduces the cost effectiveness of such systems.
Another known problem with known photocatalytic air treatment systems is fouling. Humid air streams may contain foulants that oxidize and/or collect on the surface of the photocatalyst. Coating or covering the photocatalyst with foulants therefore substantially lowers the efficiency of the photocatalyst due to a reduction in the effective surface area of the photocatalyst available for irradiation. To prevent the detrimental effects that may be brought about by fouling, the photocatalyst must be frequently cleaned or replaced. This is a time consuming and thus expensive procedure. In addition, bonding or affixing a photocatalyst to a substrate or bed reactor, and maintaining the photocatalyst immobilized, diminishes the efficiency and effectiveness of the photocatalyst. For example, the crystal structure of a photocatalyst is typically undesirably altered when a photocatalyst is heated in the course of bonding the photocatalyst to a substrate. It is also undesirable to replace an already bonded or affixed photocatalyst since such a replacement is a time consuming and expense procedure.
With known photocatalytic air treatment methods, safety issues arise when explosive ratios of contaminants exist in a given air stream. This follows since direct photocatalytic treatment can potentially ignite the explosive contaminants due to temperature elevation or the ignition source from an irradiation element.
Therefore, a need has arisen for a system and method for photocatalytic treatment of contaminated media that overcomes, among others, the aforementioned shortcomings of known photocatalytic treatment methods.
In accordance with the present invention, a system and method for photocatalytic treatment of contaminated media are provided that substantially eliminate or reduce the disadvantages and problems associated with known systems and methods for photocatalytic treatment of contaminated media.
According to the present invention, a photocatalytic process operating in an aqueous phase is employed to treat varying types of contaminated media, including air, soil or adsorbents (for example, activated carbon). For example, pursuant to the present invention, a liquid, such as water or another suitable substance, may be used to circulate and effect the decontamination of a contaminated media, in conjunction with a photocatalytic process. By transferring contaminated media to an aqueous phase, in which they are decontaminated by a photocatalytic process, significant advantages are derived.
One aspect of the present invention is a system for treating contaminated media. The system includes a converter such as a gas scrubber, which is used to transfer gaseous contaminants into a liquid media, such as a photocatalytic slurry. A photocatalytic system is connected to the converter and is used to oxidize and/or reduce the contaminants into non-harmful compounds using a photocatalytic reaction.
Another aspect of the present invention is a method for treating contaminated media. The method includes transferring contaminants to an aqueous phase. For example, airborne contaminants may be transferred to an aqueous phase by using a gas scrubber containing an aerosol, a liquid, activated carbon, or other suitable substances. The liquid media containing the contaminants is then treated by aqueous photocatalysis to reduce the contaminants.
Yet another aspect of the present invention is a method for treating contaminated media in a single vessel. The method includes injecting a contaminated gaseous media into the vessel. A photocatalytic slurry is then added to the vessel in a manner so as to adsorb the contaminants from the contaminated gaseous media, such as by spraying as an aerosol or by causing the catalytic slurry to flow counter current to the flow of the contaminated gas. The photocatalytic slurry with the adsorbed gaseous contaminants is then exposed to electromagnetic radiation, such as ultraviolet radiation, so that the contaminants may be reduced to non-harmful compounds.
The present invention provides many important technical advantages. One important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media that provides for increased mass transfer of the air phase organic contaminant and the photocatalyst, which overcomes problems associated with poor adsorption of media phase organic compounds onto the photocatalyst.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media in which the scaling of reactors for increased air flow rates can be easily accomplished by increasing the number and size of the nozzles and the size of the reaction chamber. In this manner, air treatment systems may be readily implemented that can treat volumes of air of up to hundreds of thousands of cubic feet per minute.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media that allows irreversible electron acceptors, such as hydrogen peroxide, to be added to a photocatalytic slurry containing the contaminated media in order to increase reaction kinetics and thus reduce equipment size. This method allows organic pollutants to be treated in a nitrogen environment, such as under nitrogen blankets in process tanks, where oxidants can be added to the photocatalytic slurry without creating an explosion hazard.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media in which the photocatalyst is mobilized and can be changed in-process, such as by draining the used photocatalytic slurry and adding unused photocatalytic slurry. In this manner, the treatment may be conducted continuously without requiring periodic shutdowns for replacement of photocatalyst elements.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media in which the photocatalytic assemblies are not subject to inorganic fouling with substances, such as iron and calcium. The present invention utilizes a distributed photocatalytic element that is resistant to inorganic fouling.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated air that does not require control of the humidity level of the treated air. Yet another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated media that utilizes a closed loop system that requires no continuous addition of liquid media or air.
Another important technical advantage of the present invention is a system and method for photocatalytic treatment of contaminated air that may be used to safely treat explosives (for example, nitroglycerine) or flammable contaminants in the aqueous phase rather than the air phase. In this manner, inadvertent explosion or ignition may be avoided.